This invention relates to microstrip antennas and, in particular, to such antennas having increased bandwidth.
Typical microstrip antennas consist of a flat metallic patch adjacent to a ground plane and separated therefrom by a thin dielectric substrate. Their thin construction makes them particularly useful as low-profile flush-mounted antennas on rockets and missiles since they neither disrupt aerodynamic flow nor protrude to interrupt the mechanical structure. They are also useful because of their low cost, reproductibility, design flexibility, ease of fabrication and installation and rugged design. Their unique features such as low profile, compatibility with the modular approach, ease of integration of feed lines and matching networks, and the possibility of obtaining either linear or circular polarization have made them ideal for many applications. The signal supplied to the patch may be by means of a feed conductor in the plane of the patch or a coaxial connection to an interior point on the patch. Such antennas suffer from the disadvantage of an extremely narrow bandwidth of the order of one or two percent at V.H.F.-U.H.F. frequencies and two to five percent at S.H.F. and E.H.F. frequencies.
It is known to increase the bandwidth of microstrip antennas by placing conductive strips acting as parasitic elements parallel to and spaced from the non-radiating edge of a rectangular patch or by placing shorted quarter wave-length strips parallel to and spaced from the radiating edges of such patches. This has the disadvantages of requiring significant modification to the original antenna element making it virtually impossible to use the element in an array configuration. The size of the antenna is also increased, which is also undesirable.
An alternative known manner of increasing the bandwidth of microstrip antennas is to use a linear array of patch resonators whose size and spacing increase in a log-periodic manner. At any given frequency only a few of the resonators are excited and radiate forming an active region which moves along the array as the frequency is changed.